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This page is excerpted and quoted from the National Academies of Science, Engineering, and Medicine. A Committee of over 40 experts, researchers, and reviewers in The Health and Medicine Division published a 486 page report in 2017 entitled “The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations for Research.”
If you would like to access the entire report you may do so by clicking this link.
Medical Marijuana and Pulmonary Function
“Pulmonary function refers to lung size and function. Common measures of pulmonary function include forced expiratory volumes, lung volumes, airways resistance and conductance, and the diffusion capacity of the lung for carbon monoxide (DLCO). Spirometry values include the measurements of forced expiratory volumes, including forced expiratory volume at 1 second (FEV1), forced vital capacity (FVC), and FEV1/FVC. The latter is a measure of airflow obstruction and, when combined with bronchodilator therapy, is used in the diagnosis of chronic obstructive pulmonary disorder (COPD).
Overall, acute cannabis smoking was associated with bronchodilation, but many of the authors agreed that any benefits may be offset when cannabis is smoked regularly. The current findings are inconclusive on a variety of pulmonary function measurements, and the findings may be affected by the quality of the studies, a failure to adjust for important confounders, including tobacco and other inhaled drugs, and other occupational and environmental exposures. The committee’s findings are consistent with those reported in another recent review (Tashkin, 2013) and a position statement (Douglas et al., 2015).
The majority of studies, including those evaluated in the systematic review, relied on self-report for cannabis smoking. Many studies failed to control for tobacco smoking and occupational and other environmental exposures; did not control for the dose or duration of cannabis smoking; and did not use joint-years and instead based heavy cannabis smoking on having exceeded a specific threshold of joints. Even among studies that used joint-years, it is unclear how generalizable their findings are, given the potential high variability in lung-toxic content from joint to joint. Prior studies have inconsistently documented decreases or no change in FEV1, FEV1/FVC, DLCO, and airway hyperresponsiveness. Moreover, neither the mechanism nor the clinical significance of the association between cannabis smoking and pulmonary function deficits is known beyond the possible impact of a high FVC in lowering the FEV1/FVC ratio. While elevated lung volumes could be indicators of lung pathology, an elevated FVC by itself has not been associated with any lung pathology.
7-1(a) There is moderate evidence of a statistical association between cannabis smoking and improved airway dynamics with acute use, but not with chronic use.
7-1(b) There is moderate evidence of a statistical association between cannabis smoking and higher forced vital capacity (FVC). “
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